SPC Jun 13, 2018 1730 UTC Day 2 Convective Outlook

SPC 1730Z Day 2 Outlook

Day 2 Outlook Image

Day 2 Convective Outlook  
NWS Storm Prediction Center Norman OK
1229 PM CDT Wed Jun 13 2018

Valid 141200Z - 151200Z

...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS OVER PARTS OF
THE DAKOTAS...

...SUMMARY...
Scattered severe storms are expected across parts of the northern
Plains Thursday evening through the overnight.

...Synopsis...
A compact, midlevel low, currently moving into southern British
Columbia and northwest Washington per water-vapor imagery, is
expected to track northeast from northern ID/vicinity of the
international border Thursday morning to mainly southern
Saskatchewan by Thursday night.  A 500-mb ridge axis, initially
situated over the Dakotas, will slowly progress east Thursday in
response to the northeastward ejection of an attendant mid-level
70-75 kt jet max over eastern Montana, and a deepening upstream
large-scale trough across the western U.S. through Thursday night. 
At the surface late Thursday afternoon/early evening, a cold front
will extend south along the ND/MT border and trail westward through
northern WY, while a dryline extends south through the central and
southern High Plains.  Meanwhile, a warm front should be positioned
from a triple point located in northwest SD through western IA and
northern MO.

...Northern Plains to Upper Midwest...
Although much more notable height falls (and related large-scale
ascent) are expected across southern Canada, weak height falls are
forecast over the western Dakotas mainly from Thursday evening
through the overnight period.  Meanwhile, a corridor of middle-upper
60s to around 70 F surface dew points will spread underneath a plume
of very steep 700-500-mb lapse rates.  This combined with diurnal
heating will likely promote MLCAPE values of at least 2500-3500 J/kg
across parts of the Dakotas by late Thursday afternoon.

Initial convective coverage should occur along the cold front as it
enters the western Dakotas late Thursday afternoon or by early
evening across northwest ND where initial height falls are expected.
Otherwise, an increase in low-level convergence near the triple
point in northwest SD will be another focus for convective
development by Thursday evening.  Additional storms will develop as
the cold front advances east across the western/central Dakotas,
within the residual supply of very strong instability.  Ample
effective shear will support supercells initially, capable of very
large hail and damaging winds.  The mesoscale environment for a
tornado threat should become more favorable with time and eastward
extent, given a nocturnal increase in the low-level jet.  However,
upscale growth may force less favorable convective modes prior to
the environment becoming more conducive for a greater tornado risk.

One or more convective systems may organize across North Dakota and
northern South Dakota through the early overnight, spreading a hail
and  damaging wind potential east towards Minnesota through the
remainder of the period.  Additionally, low-level warm advection may
support a few stronger cells towards daybreak Friday with southeast
extent across part of the upper Mississippi Valley.  However, the
position of the upper ridge axis suggests these storms should be
sub-severe with some small hail possible.

...MAXIMUM RISK BY HAZARD...
Tornado:   5%     - Slight
Wind:     15%     - Slight
Hail:     30% SIG - Enhanced

..Peters.. 06/13/2018

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